I have a combo box that list Customer Order Numbers. As I move up and down the list in the combo box the selection under the cursor highlights. At the point the list item highlights I would like to run a procedure that displays the description of the item as a tool tip text. Please Help.

Set the ToolTipText property of the listbox in the InteractiveChange() event.
The form's ShowTips property must be set to .T.
I assume you've already eliminated the idea of showing the description in a 2nd column of the listbox, but you could still include it in the RowSource and not show it.
e.g., If the description is in the hidden 2nd column of the listbox's RowSource.

this.ToolTipText = this.List(this.ListIndex,2)

HTH